In the formation of a semiconductor chip, integrated circuit devices such as transistors are first formed at the surface of a semiconductor substrate in the chip. Interconnect structures are then formed over the integrated circuit devices. Metal bumps are formed on the surface of the semiconductor chip, so that the integrated circuit devices can be accessed.
The properties of the metal bumps may be improved if minor elements such as germanium, silver, and/or copper are added. For example, conventional solder bumps suffer from under cooling effect, which means that the transition temperature from solid phase to liquid phase when the temperature of the solder bumps is increased is different from the transition temperature from liquid phase to solid phase when the temperature of the solder bumps is decreased. This results in a random solidification of the solder bumps, and hence the degradation in the quality of the solder bumps.